Background: Sexually transmitted infections-preventive measures are effective methods employed in the prevention of sexually transmitted infections [STIs]. Sexually transmitted infections are among the most common infections in the world and therefore a major public health problem as they cause devastating long term consequences if untreated, especially in adolescents. Sexually transmitted infection-preventive measures, if correctly and consistently used, have been proven to be efficacious. Methodology: This was a cross-sectional descriptive study carried out among SS2 and SS3 students in Nnewi North Local Government Area of Anambra state, Nigeria. A structured, self administered questionnaire was administered to consenting senior secondary students to gather relevant information about socio-demogaraphic characteristics, knowledge about STIs, sexual behaviours. Three hundred and thirty four students participated in the study. Multi stage sampling technique was used. SPSS version 20 was used to analyze the data. Results were represented in tables and graphs. Results: Three hundred and thirty four students participated in the study, of which females and males are 167 each. 89% understood the meaning of STI. The majority of the respondents got the knowledge from TV/radio (34.4%), teachers (28.1%) and mother (27.0%). The majority of respondents knew that HIV/AIDS (89.8%), syphilis (58.1%) and Gonorrhoea (56.9%) were STIs. Good numbers of respondents knew the symptoms of STIs and risk factors of STI with unprotected sex and multiple sexual partners (57.5%) and (42.2%) respectively, as the highest risk factors. About 75.7% of respondents knew that HIV/AIDS cannot be cured. The Majority knew that avoiding sexual intercourse, being faithful to one partner and use of condom are preventive measures respectively. The majority of respondents (71%) felt they cannot be infected with an STI. 92.8% believed that STIs can be prevented. Age range for first sexual intercourse was 13 - 18 years for females and 8 - 19 years for males. Of the 86 that have had sexual intercourse, 48.8% had experienced one or more of the symptoms of STIs and the majority (80.9%) went to a hospital for treatment. Males (31.4%) are more likely to have multiple sexual partners than females (4.7%). Conclusion: The majority of the students had an objective knowledge on STIs transmission and prevention. Their overall attitude was positive but their practices were not satisfactory, especially for the males. Females were more careful and health conscious than their male counterpart.